The present invention relates generally to the electrospinning of fibers, especially nanofibers, and relates more particularly to an electrospinning aid that promotes the electrospinning of one or more desired materials into fibers, particularly nanofibers.
Nanofibers are fibers with a diameter in the nanoscale range, typically from about 10 nanometers to about several hundred nanometers. Because nanofibers possess many desirable physical and chemical properties, such as a high surface-to-volume ratio and an ability to be modified and/or functionalized, nanofibers are desirable in many different types of applications including, but not limited to, textiles, membrane systems, catalysis, immobilized enzymes, chemical and biological defense, fiber-reinforced composite materials, HEPA (High Efficiency Particulate Arrestance) filters, tissue engineering, wound healing, sensors and photonics. Examples of such applications are disclosed in the following patents and publications, all of which are incorporated herein by reference: U.S. Pat. No. 6,106,913, inventors Scardino et al., issued Aug. 22, 2000; U.S. Pat. No. 6,110,590, inventors Zarkoob et al., issued Aug. 29, 2000; U.S. Pat. No. 6,800,155 B2, inventors Senecal et al., issued Oct. 5, 2004; U.S. Patent Application Publication No. US 2001/0045547 A1, inventors Senecal et al., published Nov. 29, 2001; U.S. Patent Application Publication No. US 2002/0081732 A1, inventors Bowlin et al., published Jun. 27, 2002; U.S. Patent Application Publication No. US 2002/0096246 A1, inventors Sennet et al., published Jul. 25, 2002; U.S. Patent Application Publication No. US 2002/0124953 A1, inventors Sennett et al., published Sep. 12, 2002; U.S. Patent Application Publication No. US 2002/0173213 A1, inventors Chu et al., published Nov. 21, 2002; U.S. Patent Application Publication No. US 2003/0065355 A1, inventor Weber, published Apr. 3, 2003; U.S. Patent Application Publication No. US 2003/0100944 A1, inventors Laskin et al., published May 29, 2003; and Li et al., “Electrospun nanofibrous structure: A novel scaffold for tissue engineering,” J. Biomed. Mater. Res., 60:613-21 (2002); Huang et al., “A review on polymer nanofibers by electrospinning and their applications in nanocomposites,” Composites Science and Technology, 63:2223-53 (2003); Katti et al., “Bioresorbable Nanofiber-Based Systems for Wound Healing and Drug Delivery: Optimization of Fabrication Parameters,” J. Biomed. Mater. Res., 70B:286-96 (2004).
Nanofibers may be produced by a number of different techniques, such as interfacial polymerization, melt-spinning, and electrospinning. The basic process of electrospinning was invented about 70 years ago by Formhals and is disclosed in U.S. Pat. No. 1,975,504, which is incorporated herein by reference. Additional patents relating to electrospinning include U.S. Pat. No. 4,043,331, inventors Martin et al., which issued Aug. 23, 1977; U.S. Pat. No. 4,143,196, inventors Simm et al., which issued Mar. 6, 1979; and U.S. Pat. No. 4,323,525, inventor Bornat, which issued Apr. 6, 1982, all of which are incorporated herein by reference. Until about fifteen years ago, electrospinning had received relatively little attention as a process for producing very thin fibers. However, since that time, interest in electrospinning, particularly the electrospinning of nanofibers, has increased considerably. See, for example, Jin et al., “Electrospinning Bombyx mori Silk with Poly(ethylene oxide),” Biomacromolecules, 3:1233-39 (2002); Li et al., “Electrospinning of Nanofibers: Reinventing the Wheel?,” Adv. Mater., 16(14):1151-70 (2004); Arayanarakul et al., “Effects of Poly(ethylene glycol), Inorganic Salt, Sodium Dodecyl Sulfate, and Solvent System on Electrospinning of Poly(ethylene oxide),” Macromol. Mater. Eng., 291:581-91 (2006), all of which are incorporated herein by reference.
Referring now to FIG. 1 of the present application, the technique of electrospinning is schematically shown. A quantity of a polymer solution 11 (such a polymer solution often referred to in the art as a “spin dope”) is loaded into the barrel 13 of a syringe. A needle 15 is attached to the distal end of barrel 13, and a plunger 17, which may be driven by a pump (not shown), is inserted into the proximal end of barrel 13. As plunger 17 displaces solution 11 from barrel 13, a droplet of solution 11 becomes suspended from the tip of needle 15, where the droplet is held in place by surface tension forces. An electrode 19 from a high voltage power supply 21 is in contact with needle 15 and applies an electric potential thereto, which electric potential induces free charges in polymer solution 11. These free charges, in turn, introduce a tensile force in polymer solution 11. When the tensile force overcomes the surface tension associated with the pendant drop of polymer solution 11 at the tip of needle 15, a jet of polymer solution 11 is ejected from the tip of needle 15. Fluid mechanic analysis of this phenomenon-suggests that the jet of polymer solution 11 experiences various instabilities depending upon the operating conditions and the properties of fluids. In most cases, the jet experiences a whipping instability giving rise to the bending and stretching of the jet. As the jet travels the short distance (typically about 20 cm) between the tip of needle 15 and a grounded collector 23, the contour length of the jet dramatically increases by orders of magnitude, and the jet thins to the nanometer scale. The solvent in the jet evaporates as the jet travels from the tip of needle 15 to collector 23. This evaporation of the solvent leaves dry nanofibers on the surface of collector 23. Typically, the dry nanofibers are deposited on the surface of collector 23 in the form of a nonwoven or random mat of nanofibers; however, it is also possible, for example, using a collector in the form of a rotating cylinder, to collect the nanofibers as a spool of nanofibers.
Uniform nanofibers are not typically produced from all polymer solutions. Instead, the morphology and other properties of an electrospun nanofiber may be influenced by one or more of the following: (i) properties of the polymer solution, such as viscosity, dielectric constant, surface tension, density and solvent vapor pressure, (ii) operational variables, such as the solution flow rate, the applied electric field and the electric current, and (iii) equipment variables, such as the needle size and the distance between the needle and the collector. The stability of the jet emitted from the needle depends on the viscous and viscoelastic properties of the polymer solution. Polymer solutions of low viscosity tend to produce unstable jets that break into droplets and form beaded structures, as opposed to fibers. On the other hand, polymer solutions having reduced surface tension tend to form fibers, as opposed to beads. Also, fluid mechanic analysis has shown a direct dependence of the fiber diameter on the surface tension of the polymer solution. Moreover, electrical forces are responsible for the initiation of the jet and the stretching during whipping instability. Therefore, high solution conductivity and large solvent dielectric constant tend to favor thinner fibers.
Approximately 50 different polymers, some synthetic—some biological, have been used to form electrospun nanofibers. Various different solvents, some aqueous—others non-aqueous, have been used with these polymers to produce polymer solutions suitable for electrospinning. In the past, the primary consideration that has been used in selecting a solvent has been whether the polymer can dissolve in the solvent in a large enough concentration to make the solution sufficiently viscous. (As noted above, if the polymer solution is insufficiently viscous, the jet tends to break into droplets and form beaded structures, instead of fibers.) As a result, many polymers that have low aqueous solubility or that have low molecular weight do not typically generate the necessary viscosity in an aqueous solution and, instead, have been dissolved in non-aqueous solvents, if at all. Unfortunately, however, the use of many non-aqueous solvents is undesirable from an environmental point of view, especially if one considers the large-scale production of nanofibers.